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How does an aircraft tailplane work?
How does an aircraft tailplane keep the aircraft stable, and prevent it from tipping over? Also, how does the lift generated by a tailplane compare to that generated by the wing?

A question from the early days of this site, many answers could do with an update. References to having to have negative lift should be passé by now.

For conventional designs, the tail is composed of two parts: the horizontal tail and the vertical tail. They play a role in the trim and the manoeuvrability of the aircraft but at different levels. The horizontal tail is mainly used for longitudinal stability (and trim) while the vertical tails used for the lateral stability (and trim).
About Stability
It is possible to talk of stability only after having defined an equilibrium point around which the stability is studied. An aircraft is in equilibrium if the forces and moments it experiences are balanced. Using a simple model for the longitudinal analysis, it can be decomposed in three relations called the trim equations. In order to keep it simple, it will be assumed here that the angle of attack and the flight path angle are zero. (Note that the same reasoning can be achieved with non-zero values but the equations then become quite messy.)
Longitudinal Equilibrium
These three equations are:
$$L=mg$$
$$T=D$$
$$M=0$$
where $L$ is the total lift, $mg$ is the weight of the aircraft, $T$ is the thrust, $D$ is the drag and $M$ is the pitching moment around the centre of gravity of the aircraft. The second equation won't be studied further since it does not help to understand the role of the horizontal tail and its influence. Looking at the following picture, one can see that usually, the centre of gravity and the point where the lift applies (called the aerodynamic centre) are not the same. This means that the lift generated by the wing creates an induced moment around the centre of gravity that one should add to the already intrinsic pitching moment due to the main wing (usually a pitch down moment for conventional airfoils).

Knowing that, it is possible to rewrite the two equations of interest including the contributions from the main wing and from the horizontal tail.
$$W+L_t=L_w$$
$$M_0+bL_t=aL_w$$
From these equations and the figure, it appears that the horizontal tail is used to generate a lift which induces a moment helping to balance the moments equilibrium and thus prevent the aircraft to spin on itself (pitchwise).
Drawback and Solution
From both the figure and the equations it turns out that the lift contribution from the tail is usually negative, meaning that more lift from the main wing is needed to keep a trimmed (or balanced) aircraft. This drawback can be overcome by the use of a canard configuration instead.
Lateral Stability
The same can be done for the lateral equilibrium and stability but there it is the vertical tail that is used. It is symmetrical so that there is no yaw induced and if there is some side force experienced, it will create a moment in order to reduce the side-slip angle.
Comparison of Lift Created by the Tail and Main Wing
For a trimmed configuration, it is easy to see that the lift created by the main wing is more or less the one created by the tail plus the total weight of the aircraft, which gives an idea of the difference between the two forces.

There is nothing really wrong with the existing answers, but I feel that they don't really dig down to the core of the issue. But it is actually not so complicated ...
All that is required for static longitudinal stability is a lower lift per area on the horizontal tail than on the wing. Downforce on the tail helps, because then the lift at the tail is obviously lower than on the wing, but is not necessary. What counts is that the relative lift change at the rear lifting surface due to a change in the angle of attack of the whole airplane is higher than the relative lift change on the forward lifting surface. The mechanism is the same for conventional configurations, canards or even flying wings.

Say the airplane flies at the angle of attack $\alpha_1$ and is disturbed by a gust or a sudden control input, such that it assumes a higher angle of attack $\alpha_2$. Due to camber and a higher incidence, the lift curve of the wing (blue line) is shifted up relative to that of the tail (green line). Also, the downwash effect and the lower aspect ratio reduce the lift curve slope of the tail relative to that of the wing.
Now assume that the aircraft was trimmed in state 1, such that the moment from the small tail lift was equal to the moment of the much larger wing lift around the center of gravity. In state 2, the absolute lift change ∆L on the wing is much smaller relative to the lift at state 1 than on the tail, such that the resulting moment change produces a pitch-down moment. The same happens with a reduction of the angle of attack in state 2, only in reverse.
$$\frac{∆L_{Wing}}{L_{Wing}} < \frac{∆L_{Tail}}{L_{Tail}}$$
If the lift ratios would be equal for wing and tail, the balance of moments would not change between state 1 and state 2. But since the tail experiences a higher relative lift change, a moment change follows which works against the change in angle of attack.
This effect also works for a canard, where the lift per area on the foreplane needs to be bigger than the lift per area on the wing. For a flying wing the the lift per area of the forward part of the wing needs to be bigger than that on the rear part of the wing, and still static stability is possible.

A wing with a conventional aerofoil profile makes a negative contribution to longitudinal stability. This means that any disturbance (such as a gust) which raises the nose produces a nose-up pitching moment which tends to raise the nose further. With the same disturbance, the presence of a tailplane produces a restoring nose-down pitching moment, which may counteract the natural instability of the wing and make the aircraft longitudinally stable (much the same way a windvane always points into the wind). (From the Wikipedia page on tailplanes)
The tailplane does not produce any lift. You could say it produces a 'Negative Lift'. The reason many early aviators were killed is because the tailplanes produced lift in order to help the plane fly which would result in unrecoverable nose-up tailplane stall. Most modern aircraft are designed so that when the airflow decreases, the effect/momentum produced by the tail surface is decreased as to prevent the previously mentioned condition

According to a book about the Wright Flyer, early planes were deliberately designed to avoid having them nose-down into a stall; that meant that stalls could not be recovered in the air, but tended to limit the speed at which the planes hit the ground.  The first air fatality was a result of a broken control cable, which caused the plane to nose down into the ground, hitting it quickly, rather than stalling and hitting the ground slowly.

@supercat: The first air fatalities were Pilâtre de Rozier and Pierre Romain. The first heavier-than-air fatality was indeed caused by a stall and the Wrights chose the canard configuration in the mistaken belief this would make just this kind of stall impossible.

according to how it flies, sec 6, the tailplane does not necessaraly produce "negative lift". It only needs a lower AoA.

@PeterKämpf: You mean it would make "nose-down" stalls impossible?  The book suggests that they recognized that their design would create unrecoverable stall situations, and stalls were a frequent occurrence, but the first (and I think only) fatality in a Wright plane of that design occurred when a control linkage broke (which would be bad news in a plane of almost any design which lacked redundant control mechanisms).

@supercat: No, it does not make  "nose-down" stalls impossible. What prevented them from happening to the Wrights was their choice of the center of gravity - all early Wright flyers were statically unstable, and stalls would first occur on the main wing. By quickly commanding a nose-down moment with the unstalled and fully functional canard they could recover every time.

@PeterKämpf: We both agree I think that a canard with the properties of the Flyer will not gain speed after a stall; would it be fair to describe its stall behavior as entering aerodynamically-stable envelope whose control range is insufficient to allow a return to sustainable flight?  My intuition would suggest that it would be harder to make a rear-tail plane stable in that way, and that it would be more likely to pitch up so much that it would then fall backward.  Would that not be the case?

@PeterKämpf: To be fair, having a canard configuration does make for an unstallable aircraft... iff the canard is at a higher angle of attack than the main wing even at its maximum nose-down position.

The wings(which has a aerofoil cross section ) produce lift (basically a force that acts opposite to weight)  which act at a distance from center of gravity (C.G) so the force gets transferred to C.G. as a force and moment(in clockwise direction) which lead to  pitch up movement
To balance that moment tail are used the tail produce lift (small compared to that produced by wings) so if we transfer it to C.G. an force and moment (since it produce less lift it should be placed far from C.G)  this moment acts in anticlockwise direction thus neutralizing the moment due to wings ...Thus making the aircraft stable ...

The absolute value of the lift generated by the tailplane varies and depends on the phase in which your plane is in that moment:

Takeoff (flaps extended): high drift
  Climb (no flaps): mostly lift (not much)
  Cruise (no flaps): drift
  Landing (flaps extended): high drift

Due to the fuel consumption the weight of the plane reduces while flight. This may change the position of your center of gravity and this in turn will affect the absolute value of your lift/drift. Usually |drift| increases, in other words, while flight the lift of the tailplane decreases.
Some words to stability:
Just think about equilibrium of moments.
The center of gravity is near the main wing. The high lift of the main wing is very near to c.o.g., the drift of the tailplane is rather far away from it. The sum of all moments equals zero, they will balance the plane if there are gusts etc.


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Are there any twin engine aircraft without counter-rotating props?
All the twins I have checked have counter-rotating blades, so I was wondering if this was true for all twins of if there are any twins that have both blades turning in the same direction.

Almost all WW2 twin engine aircraft use standard props and therefore do not have counter rotating props.

Twin enigned aircrafts without counter-rotating propellers
A great many twins don't have counter-rotating props:
One such aircraft is the BeechCraft Baron 58, as seen in this YouTube video.
Another aircraft I can think of is the King Air 350.
It is not any more or less common to have one or the other.
Contra-rotating propellers
Another interesting solution to the P-factor (prop pulling towards one direction), especially on single engine aircraft is contra-rotating props, two propellers on the same shaft that spin in opposite directions.
This is mainly for single engine racer aircraft, but there are multi-engine aircraft known to have it, such as the Soviet Tu-95 Bear bomber.
The pic shown here is a Spitfire Mk. XIV:

A better explanation of the P-factor
P-factor is the term for asymmetric propeller loading, that causes the airplane to yaw to the left when at high angles of attack.
Assuming a clockwise rotating propeller it is caused by the descending right side of the propeller (as seen from the rear) having a higher angle of attack relative to the oncoming air, and thus generating a higher air flow and thrust than the ascending blade on the left side, which at the other hand will generate less airflow and thrust. This will move the propellers aerodynamic centre to the right of the planes centreline, thus inducing an increasing yaw moment to the left with increasing angle of attack or increasing power. With increasing airspeed and decreasing angle of attack less right rudder will be required to maintain coordinated flight.

ah, you're right. I edited my post ;)

DA42 also has both engines turning in the same direction

Another interesting issue that effects aircraft with non counter rotating props is that they would have a Critical Engine. This in basic terms means that it's worse if a particular engine fails. The Diamond DA42 which is a popular modem trainer has this issue if the left engine fails.
The following from Wikipedia explains further.
Critical Engine

The critical engine of a multi-engine, fixed-wing aircraft is the one whose failure would result in the most adverse effects on the aircraft's handling and performance. On propeller aircraft, there is a difference in the remaining yawing moments after failure of the left or the right (outboard) engine when all propellers rotate in the same direction due to the P-factor. On turbojet/ turbofan aircraft, there usually is no difference between the yawing moments after failure of a left or right (outboard) engine. An engine can also be called critical when it is the only engine that drives a hydraulic pump for augmenting/ boosting flight controls.

An interesting thing about the P-38 Lightning is that it had TWO critical engines. The initial prototype had no critical engine, but later the prop rotations were reversed to improve airflow over the tail.

Yeah. It mentions that in the article linked above. Apparently to make it a better gunnery platform.

Piper Twin Comanche was available in both configuration, but the most popular one had the engines turning in the same direction.  Aztec and Apache are also models with engines spinning the same way.  My experience is that most light twins have the same direction and that counter rotating is the exception.  It makes sense that way as they share engines with singles, so one particular spinning direction is going to be much more common and spare parts will be easier to get...

source: Stack Exchange

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Why can't most jets safely use reverse thrust while airborne?
According to Wikipedia:

Some aircraft are able to safely use reverse thrust in flight, though the majority of these are propeller-driven. Many commercial aircraft cannot use reverse thrust in flight

What are the repercussions to using reverse thrust while airborne?

The wonderful Aerospatiale-BAC Concorde could use its reverse thrusters in flight to make 10000 fpm descents without overspeeding (I must've mentioned the Concorde in almost every one of my posts on Aviation SE XD)

The problems are two-fold:

The airspeed is much higher in flight than when landing, so the thrust reverser doors would have to be much stronger, and therefore heavier to withstand the additional forces, along with the relative wind making them harder to close.
What happens when it gets stuck in the deployed position?  Now you have to shut down a perfectly good engine and have additional drag on the airplane while in flight.
(Bonus) It isn't really needed on most airplanes with spoilers, flaps, etc. that accomplish the same thing.

The reversers on many jets significantly disrupt the air flow around the wing and are generally too powerful for use in flight.
There was a crash, Lauda Air Flight 004, that was caused by uncommanded reverser deployment in flight. Granted, it was asymmetric deployment, but the report cites loss of lift from reverser deployment (itself, not loss of speed due to it's prolonged operation), which would occur for symmetric deployment too.

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Is there a practical way to demonstrate sustained flight on the back side of the lift curve without an AoA meter?
Here is a $C_L$ / $AoA$ curve that I took from Wikipedia.

The better textbooks say that a stall is that condition in which a further increase in angle of attack will result in a reduction of lift.  The point at which that transition happens is known as the critical angle of attack.  Theoretically, sustained flight is possible at angles beyond the critical angle of attack - take a look at the chart.  If the airplane can sustain level flight at point $A$, it can sustain level flight at point $B$.
Is there a practical way that I can demonstrate sustained flight on the backside of the lift curve without an angle of attack indicator?
Or to ask the question another way,
Is there a practical way to tell when an airplane has exceeded the critical angle of attack without an AoA meter?

just nitpicking, but drag is much,much larger in B than in A. If you have enough engine power for sustained flight at A, it doesn't imply you can sustain straight & level flight at B. Plus B is statically unstable as DeltaLima has answered already

Flight on the backside of the stallcurve is unstable; any (infinite small) increase in AoA will result in less lift. Less lift will cause the flightpath angle to decrease and the AoA to go up even more. To increase the lift, the nose has to be pitched down, but not so much that the stall is recovered otherwise you wouldn't call it sustained flight at the back of the AoA curve.
At the same time you are also on the backside of the powercurve. This means that any reduction in airspeed would cause the drag to increase therefore decelerating the aircraft furthermore.
This double unstable situation would require a lot of fast responses from the pilot, probably causing too high workload. I doubt if it is practically possible to fly on the backside of the liftcurve without special automation.

Just pull all the way back and let the plane do the rest. Depending on the airfoil, this may or may not work. If stall is characterized by flow separation near the nose (think NACA 5-digit airfoils), the aircraft will pitch down. If you use a modern laminar flow airfoil, chances are good that you can trim this without problem. The missing thrust will be compensated with sink speed. Air France demonstrated that you can keep an A-340 comfortably in this condition for 3 minutes, and the automatization was switched off.

@PeterKämpf I agree with your analysis. A steady spin is another a demonstration of remaining in this B state. In that case the high wing will be a bit to the left of point B, the low wing a bit to the right, but both will be passed the top. But I don't think this categorises as sustained flight though, as AF clearly demonstrated. (And the nitpicker in me says it was an A330, it was not comfortable, and the automation was no switched off but it was in Alternate Law)

@DeltaLima +1 for the last 2 sentences of your comment. Also, I believe it was more like 6 minutes. Definitely not sustained, though.

@DeltaLima you don't have any glider time?

@rbp 45 minutes or so.

prolly not enough to have spent a lot of time on the backside of the lift curve

@rbp Just saw your answer, I guess that's the perfect excuse to get some more glider time next summer... :-)

Hm, I don't think the powercurve statement holds. Drag doesn't increase because you slow down; it increases as a consequence of maintaining the same lift. But at this point, to maintain the same lift you'd need to decrease AoA, which would reduce drag.

Gliders make sustained flight on the backside of the lift curve all the time while thermalling at 40-50 knots and 50 degrees of bank. You spend a lot of time adjusting the elevator to keep the plane from stalling, a lot of time adjusting the ailerons to keep the bank angle, and a lot of time trying to stay coordinated (or in a slight slip) with your feet.  The bigger the wingspan, the more work it is.
The big difference in a glider is that you're usually going up, not down when this happens :)

Increase the aircraft's pitch in level flight. If you climb, you are on the good side of the curve. If you sink, you are on the bad side. This will only be the initial response of the aircraft, as the airspeed will start to change and further affect the aircraft's behavior.
Without an AOA meter, you would have to look at your artificial horizon and vertical speed.

If you sink you are not necessarily on the backside of the liftcurve. Your airspeed will be lower than it was in level flight and your AoA will be higher, but not past the critical AoA

If you increase the pitch, you'll initially increase the lift and the aircraft will start to climb and loose airspeed. Less airspeed is less lift so the climb will shallow out over time and might develop into a sink depending on pitch, power and dynamics of the aircraft.

Is there a practical way that I can demonstrate sustained flight on the backside of the lift curve without an angle of attack indicator?

Set up the aircraft for stalls, do all the stall safety checks. Fly slowly, so that you're near the peak of that curve. Pitch up sharply and confidently while adding power. When you've done the right amount of both, you'll find that the aircraft remains more or less in level flight. Make adjustments to zero the vertical speed, remembering that if you're climbing, you want more up pitch to stop it in this configuration, and vice versa.
P.S. I'm no instructor and my flying hours are limited, but I was taught to do this during my initial training. I suppose how easy this is depends strongly on the aircraft.

The drag at point B in your chart is much higher than at point A, so you might not be able to trim level flight there. If you are willing to sacrifice altitude to compensate for missing thrust, the lift at point B will be sufficient to trim a quasi-steady flight condition.
Flying beyond stall requires sufficient pitch authority, so you might not be able to trim this condition with forward c.g. That said, your pitch response will be much weaker than in normal flight, and you can pull suddenly without much happening. Depending on your c.g. location, you will need to have your yoke/stick/whatever pulled way back to trim this condition, and if some more travel in pull direction remains, it will not help to further increase your angle of attack. Some aircraft will not let you stay in this condition and either drop the nose by themselves (which is good) or drop a wing (which is really bad), so try to experiment with enough altitude for recovery.
My answer is Yes, if the aircraft lets you. Just watch the sink speed.

Lift coefficient over 180° for several NACA airfoils, taken from Sighard Hoerner's Fluid Dynamic Lift.
@DeltaLima is absolutely correct: While the lift curve slope is negative (the region around point B in your plot), the aircraft will be unstable and will drop out of this alpha range quickly. Note, however, that in the post stall region another section with positive lift curve slope follows, and here you can fly (well, let's better say mush) stably if the pitch control power of your aircraft allows. You will detect that you are in this region when your sink speed is much higher than at around stall, but the aircraft is in a stable state.
Air France demonstrated that you can keep an A330 in this condition for three minutes without any trouble. The trouble only starts when your altitude has run out.

Miss clicked on the down vote yesterday, noticed today, can't change anymore. sorry.

@Antzi: Thank you for letting me know!


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What is the function of the scissors link in helicopter flight controls?
What is the function of the scissors link on the rotor of a helicopter?
I assume it has something to do with the flight controls, but I'm not quite sure what.

@yadi I've edited your question to be a little less vague (and to match up with the answer). If you were talking about some other scissors link in a different flight control system please open a new question, with more detail :-)

the scissor link ensures the upper swashplate rotates with the rotor blades (and the lower with the body) while ensuring enough movement for the plate to control the craft (tilting and up and down).

(http://en.wikipedia.org/wiki/File:Robinson-Rotorhead.jpg)

To keep the stationary swashplate from rotating with the rotor, a scissor mechanism is added. The scissor connects the swashplate to the fuselage. The scissor consists of 2 parts which are connected by a hinge. One end is connected to the helicopter using a hinge, while the other is connected using a ball joint. This means that the swashplate can move freely up and down, and can freely tilt in any direction, but can't rotate around the mast.[source]

this is needed because the control linkage is typically attached with balljoints which would otherwise allow for too much relative movement.


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What do the Thunderbirds pilots do when they can't fly?
I went to an airshow recently, and the Thunderbirds performed. However, by the time they finished the startup and flight check ceremonies, bad weather had rolled in. However, they did still taxi into position at the end of the runway to wait for the bad weather to blow over. About 45 minutes to an hour later, they decided visibility was to bad to fly, and they taxied back to the ramp.
My question is if there is a solitaire game built into the Thunderbird flight software or something to keep them from dying of boredom, or if they just stoically stare at the end of the runway.

stoically stare at the runway I guess, or banter on the radio

Would you be willing to say what airport they were at?

What does any pilot do while on the ground, in the cockpit, waiting for weather to clear (or doing something else other than making a go/no-go decision)? This has nothing to do with Thunderbirds.

@CGCampbell It was KYNG.

@mah read a book?

What do you do when you're stuck on the interstate for 45 minutes due to an accident up ahead? Look around, twiddle your thumbs, maybe listen to the radio a bit? Same thing exactly. Possibly easier for them because they're getting paid for it. Definitely no games on the HUD.

@dvnrrs I was kidding about the games in the HUD. ;-) However, unlike someone stuck on an interstate, they did not have to look out for traffic moving or anything. They just sat there, perfectly still. If I were parked outside a store for 45 minutes I would use my phone or read a book

@flyingfisch They're professionals. They probably do get bored sometimes, but it goes with the job. Keep in mind they're all seasoned combat pilots. Long missions can get very tedious. 45 minutes at the end of the runway is far from the worst those guys have had to endure. But they likely wouldn't trade it for anything.

Can't you just picture playing Flappy Birds on their iPhones?

I would imagine they do the same thing all pilots do when they go to the airport expecting a nice day and wind up grounded by weather: Sit around at the airport with the other pilots and complain about the lousy weather. (In this case, since they're all sitting in position waiting for the weather to clear up any griping was probably done over the radio.)
There are of course other things they can do, being a fairly well-known demonstration team and an important component of the public relations arm of the US Air Force, for example signing autographs and talking to spectators at the airport.

I suspect they don't gripe on the radio - too many spectators have scanners & it would spoil their image.

@DanPichelman Gripe on UHF - most folks only have VHF radios/scanners :-)

Makes sense to me :-)


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How do you repair a grounded aircraft at a remote location?
If a large passenger airliner needs repair at a remote airport without services, are there "flying repairshops" that can swoop in and do the work on site?

...like this! :) (OK, maybe the DC-3 isn't "large" by today's standards...)

If you can, you'll fly in maintance yourself to get it fixed, perhaps by chartering a cargo plane on the way. If you can land the aircraft, you can probably get any spare part in through the same runway, with varying degrees of difficulty.
If it's bad, Boeing has an Aircraft On Ground team who should be able to fix pretty serious damage. Often, it may just be patched up until it's good enough to fly to get it to a place with better facilities.

It seem airlines also share technical resources including their own 24/7 AOG rapid response teams. There are also independent companies offering this sort of service. Example 1, example 2

yup. KLM used to have (maybe still does) a regular shuttle flying maintenance rigs, spare parts, and crews to stranded MD-11s. Maintenance facilities for them are few and far between and they were/are rather prone to trouble in the #3 engine (of course, always the hard to reach bits fail first).

@jwenting: #3 engine is accessible reasonably. It's under right wing. Engines are numbered left to right, so the hard to access centre one is #2.

If the problem is a bad engine and the aircraft is a 747, the most cost effect solution is usually a 3-engine ferry to a repair station. From memory, the protocol to be observed includes:

The Captain must do the flying.
Only the cockpit crew can be aboard.
The fan must be tied down to prevent it's rotation in flight.
A specific takeoff procedure is to be followed.

At the two 747 carriers I worked for, we usually did a 3-engine takeoff in the sim on our 6-month checks. I did two of them for real, and it's no big deal.

No airport capable of handling widebodies will have no services at all, but of course many places lack a full maintenance shop.
As mentioned elsewhere, Boeing has a go team that can get most repairable damage brought back to flyable condition. "Flyable" does not necessarily mean "normal, scheduled service" - it means able to leave the remote field and limp back to a facility able to do the proper work. This could easily be Seattle or Hamburg from Johannesburg. If you want to reposition an aircraft for maintenance you can have a LOT of stuff broken - basically if the engineers say they'll ride in it, and you avoid overflying large cities, you're good.
Several large aircraft have an option to carry an extra engine on the wing to a remote grounded aircraft - expensive, but cheaper than cargo (it can be on a scheduled revenue flight). Put a couple of mechanics on board and the job's done in a day or two. The bad engine comes back the same way.

Actually, quite a few do lack the facilities because no such aircraft are stationed there, they just land, drop off passengers, and leave again with hopefully more passengers. Think charter destinations. Or what facilities they have are for light aircraft and maybe business jets only, incapable of handling anything airliner size. That's in fact very common.

@jwenting Things are hopefully better now than they were in the 1990s when I was captaining 747s, but we constantly faced the problem of having no repair capability (or even working huffers if the APU was inoperative) in third world countries, and especially in Africa. Right or wrong, we often didn't write up grounding problems in such places when there was no real compromise in safety.

@Terry I doubt much has changed in most of Africa except for the worse in the last 20 years or so...

There are also issues that arise when an airplane has to divert sure to an emergency, and maintenance may not be available where they land.

@nitpickers: when I said "no services at all" I meant it. Nothing. No FBO, no terminal, diesel is delivered by truck only on request. You don't land a widebody there unless you're desperate.

That would depend on the damage that needs to be repaired.
Some repairs (like replacing an engine) can be done on site.
For more serious damage that can't be fixed on site they will make a quick patch and then fly the aircraft to a facility capable of properly doing the repairs while taking care to not aggravate the existing problem.


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Why are vertical stabilizers always at the rear of an airplane?
Sharks and whales have them in the middle of their body, so why not planes?

because planes =/= sharks and whales

The Wright Flyer had it's horizontal stabilizers at the front (like modern canards), but the vertical stabilizer/rudder was at the rear. http://upload.wikimedia.org/wikipedia/commons/f/f9/Wright_Flyer_photo_D_Ramey_Logan.jpg

It's the difference between aerodynamics and aquadynamics. Flying and swimming are not as comparable as assumed.

Not all of them have the vertical at the rear - in fact, the Pterodactyls used active stabilization with their long snouts and the fin at the rear of their heads. Also, the Hotol project proposed a forward fin with active control.

With sea creatures the dorsal fin acts as a keel providing the actual sideways force during the turn. They remain level while turning.
Planes on the other hand bank to turn where the sideways force is provided by the lift of the wings.
To provide yaw stability the vertical stabilizer should be as far from the center of mass as possible. They are not in front because that would be unstable as the minor deviation from straight ahead would increase the force into the deviation rather than provide a force to counteract it.

Exactly. Having the vertical stabilizer in the rear makes the aircraft behave much like a wind vane. If one side of the aircraft or the other is facing the airstream, the air will push against the vertical stabilizer, forcing the nose to point back into the wind. As racket freak said, this provides stability in the yaw axis. For aircraft that are designed to be stable (as opposed to, say, fighters and aerobatic aircraft,) aerodynamic stability is designed into the airframe in all 3 of the rotational axes.

This is one of those answers where I wish there was a video. Nice answer nonetheless.

@reirab - most modern fighter aircraft are designed to be unstable, as it helps with agility. Stability is provided by computers, who can correct the instability much faster than a human pilot

@JonStory Yes, that's why I said "as opposed to, say, fighters and aerobatic aircraft." Both of those classes of airframes are typically designed to be unstable for increased maneuverability and rely on the pilot and/or computers to provide stability.

Apologies, I must have misread your post

The vertical stabilizer is for "stability". By having it aft of the center of gravity, it will generate a side force that will correct out any undesired yaw.
Try throwing a paper airplane or a dart backwards, and watch how it flips around due to this stabilizing force to have the stabilizer in the rear.

To add to other answers, vertical stabilizers also have control surfaces on them, and in order to maximize the effect of those control surfaces, you have to have them as far out from the center of horizontal rotation as possible.
Why not in front:
Stabilizers create drag. Drag is pulling backwards. When they are in the back, that's a natural resting point for them and thus they actually try to keep plane stable in one direction. Move them to the front and you would get a force that constantly tries to turn you butt-first :)
Example you can try at home:
Imagine a swinging door. The closer you move the door handle to the hinges, the more force is required to open the door. same applies to the airplane. If you place the rudder in the middle it will mostly push plane to the side instead of turning it.


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How exactly is the 51% rule applied for amateur-built aircraft?
I understand that for an amateur-built that you are required to build at least 51% of the aircraft yourself, but how can there be kits which appear practically completed, just some quick assembly jobs? Could someone explain how this 51% rule works, and what is this percentage based on?

The requirement which states that at least 51% of the tasks needed to make the aircraft airworthy must be accomplished by amateurs is 14 CFR 21.191(g). Presumably this would exclude finishing items like paint, some instruments, avionics, interiors, etc., as those are items which in most cases are not required for airworthiness.
The FAA has several checklists which inspectors use to evaluate the "major portion" requirement. Homebuilders thoroughly document the construction process (or at least, they should!) in a builder's log; a lot of these tend to turn into massive photoblogs. AC 20-27G contains guidance on what this documentation should look like and contain.
The "two weeks to taxi" builder-assist programs that exist out there generally get the basics working, but they certainly aren't "two weeks to flight". Most manufacturers carefully calculate the amount of work they can offer as a kit, based on the number of tasks remaining for the builder to finish.
I'm not a builder, so I can't say for sure, but I understand most of the advantage to these programs is not that somebody is helping you - it's that they can supply tools, jigs, and a lot of behind-the-scenes support like organizing your workflow and providing answers to questions with no turnaround time. They're not there building the plane with you; they're there to make it possible for you to build the plane more quickly... if that distinction makes sense.
EAA also offers a pretty good FAQ for the 51-percent rule, which I admit to paraphrasing here and there!

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Passenger window insert pops out mid-flight
I was on a domestic flight from DCA to ORD in December on an Airbus A319. I was in row 31 and two rows ahead of me on the left side of the plane, I noticed while deplaning at ORD the passenger window insert was partially popped out. It was pushed out by about 2-4 inches at the top angled inward and near flush at the bottom. I didn’t hear a noise in flight and as the passenger in that row was leaving I asked if he noticed it mid-flight and he said no. The outer window on the skin of the plane looked undisturbed with no cracks fortunately. I informed the flight attendant upon leaving to make sure maintenance looked at it right away.
Today’s story of the Alaska Airlines 737-9 Max losing its side panel and window between PDX and ONT reminded me of it.
Different airframe, but what could have happened if my flight’s window had failed? Would the localized damage resulted in losing the entire panel as was shown in press reports for the 737-9 Max? For my flight, would it have been an easy fix of popping it back in or do you think they grounded that plane and taken it to the hanger for a more serious repair and inspection?
Subsequently, based on a helpful comment and some more sleuthing. I can confirm that the window was clamped in vice bolted and the appearance was similar to this video on an A320neo interior.
You can see the frame around the window was the piece protruding out.

It’s worth noting that in the Alaska Airlines 1282 incident, it isn’t a part of the fuselage which got ripped away, and it’s unlikely it had anything to do with a window: it was an extra emergency exit, supposedly factory plugged, which failed. You can very clearly see the shape of the door from the inside, and even more from the outside.

blancolirio covered the Alaska Airlines 1282 one.

The link at the bottom of your post is just a link to your google search results, it does not provide any specific video. Do you have an accurate link to the video you intended?

Here’s a link to a video with the same windows. https://youtu.be/cAgAXRgwKfQ?si=BeY8o3VYuf2nSXG0

Copied from https://thepointsguy.com/news/what-are-airplane-windows-made-of/:

A cabin window consists of three panes: 1) an outer pane flush with the outside fuselage, 2) an inner pane — which has a little hole in it you may have spotted, and 3) a thinner, non-structural plastic pane called a scratch pane. Passengers can't touch the inner pane (the one with the hole in it) or the outer pane, for safety reasons. Instead, passengers can rest their weary heads against the scratch pane, press their iPhone against it, or simply muck it up with greasy fingers. The scratch pane isn't actually part of the window assembly itself, but installed separately.

So it looks like it was the innermost scratch pane that was loose. So as long as the other panes remain intact, there was not danger, although I wouldn't recommend removing the scratch pane. Since it is installed separately, it was probably closer to "popping it back in" than a major repair.

The repair probably involves popping out the inner trim ring, possibly by undoing a few screws, putting the scratch pane back in, the replacing the trim ring. The trim ring might just be a press-fit affair...

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Why tap the brakes on take-off in fixed gear aircraft?
The Socata TB10 checklist indicates that you should touch the brakes after lift-off. I assume this is to stop the wheels from turning, but why?
In a retractable gear aircraft this would make sense to avoid using anything in the wheel-well to stop the wheels and potentially causing damage or generate heat (as a side note, I think some airliners actually have brake-pads in the wheel wells for this purpose). However, the TB10 is not equipped with retractable gear.
The TB10 and TB20 are very similar, and it might be a case of copy & paste between the two POHs (the TB10 POH also lists V-speeds for "flaps and landing gear retracted").
Do spinning wheels have an adverse effect on aerodynamics (or handling in general)? I would assume, if anything, they'd produce less drag. And as a cherry on top, I'd also assume they'd start spinning again fairly quickly, as only the lower part of the wheels are protruding out in to the air flow.

An instructor I sometimes fly with constantly taps the brakes just after take-off when we fly in my 182. His reason is mainly out of habit; he's a former regional carrier ATP.

depending on the condition of the runway (grass, dirt) and tires, rotating wheels can induce a little shaking in a DA-20. As a rule, I always step on the brakes after lift off to stop that annoying vibration

I'll tap the brakes in the Cherokee if I feel a vibration that I think is related to the gear (though the one you're most likely to feel is the nose wheel, which has no brake...) - If that stops the vibration I know the main wheel bearings may need attention.

Maybe it's in case you install an STC for retractable gear?

This answer to another question has a link suggesting that the spinning wheels may be sufficiently large gyroscopes to affect handling of the aircraft. That was however on a larger aircraft and higher speed (Lockheed Constellation). I don't know whether that effect would be observable on TB10.

On a Cessna 172, spinning wheels do create vibrations. Once the brakes are hit the vibrations stop. It could be due to the wheel balancing.

This would be my answer. To stop the vibrations that can especially agitate unknowing passengers.

@user2168 - get in a C172, ask the pilot not to touch the brakes. Once you feel the vibrations, ask him to tap the brakes, note that the vibrations stop. Voila, documentation.

Spinning wheels did cause vibrations on the 172s I flew. (Slightly out of balance wheels, I suppose.) It's always good to eliminate benign causes of noise, so they don't potentially mask noises that require attention. It's always good to know the cause of any noise or vibration.

My initial hunch tells me that the reason for this is a bit of angular momentum.
Remember back when you were a kid, first learning to ride a bike? At some point you realized that the more speed you have, the more stable you are on your bicycle. This is because of angular momentum.
So imagine your up in the air, and you have to wheels rotating at 75 rpm (assuming a takeoff speed of 55KIAS, no headwind, and a tire diameter of 18 inches). You have a decent amount of angular momentum fighting any maneuver that the aircraft wants to make.
Consistency is key in safety in aviation. The difference in ground speed for a takeoff could vary from the Vr KIAS ± max head/tail wind. That's enough to significantly alter the dynamics of a maneuver.
--EDIT--
If you want to know how much you are affected by this angular momentum, check out the question I posted on the physics stack exchange.
https://physics.stackexchange.com/questions/111353/what-is-the-inertia-caused-by-angular-momentum-when-twisted-on-its-rotating-axi/111422

To the person who downvoted: why? Care to share?

Actually, gyroscopic effects make very little contribution to the stability of bicycles, as shown by experiments by David Jones in the late 1960s. For more details, see Lowell and McKell, *The Stability of Bicycles (American Journal of Physics, vol 50, no 12, pp. 1106-12, 1982; PDF).

Physics SE on the subject of bycicle stability: little to do with gyroscopic inertia.

Spinning wheels are not a problem in fixed undercarriage aircraft, there's no good reason to tap the brakes after takeoff if the gear is nailed down. I suspect you answered your own question when you theorized that they may have simply copied that part of the manual.
From a wear perspective you may save some wear on the bearings by stopping the wheels spinning, but since you are using the brake pads to do it it's likely less overall wear just to let them stop naturally.
CLimbout is a busy time, anything that adds workload without a benefit shouldn't be on a checklist. I'd say you are probably safe to ignore that in a TB-10, however that is just my opinion and it may be worth contacting a TB-10 club and asking them what they think.

It's strictly done to stop the wheels from vibrating as they wind down, which can scare the pax and sometimes the pilot at first. Light aircraft wheels don't get any kind of post-install mass balance until they get to a certain size (and even large wheel assemblies on airliners are only statically balanced in a static rig they way motorcycle tires are done - you won't find any automotive style wheel balancers at an aircraft shop).
For light aircraft tires, there is a marked light spot (a yellow or red dot) on the tire that you put next to the air valve when you mount the tire in the wheel halves.  That gets you fairly close to balance, but they are never perfectly balanced, and this isn't really a problem except when they are spinning freely on the spring of the gear leg, where they will vibrate in phase as the spin rpm matches the natural frequency of the gear assembly slowing down.  This is most pronounced on spring steel gear legs.
Airliners do it automatically on the main gears by applying the brakes (vibrating main gear tires on spin down would really terrify the pax!), and nose wheels are done by snubber pads in the gear well that rub on the tread as the wheels come up.  It's not unusual for flight crews to snag nose gear tires that vibrate like crazy before the snubber blocks stop the wheels.

I think you have the red dot info backward. "Set the outboard half in the tire. Align red-dot on tire with the wheel valve or wheel's heaviest point if indicated.
Align bolt holes in accordance with manufacturer's instructions." https://www.bridgestone.com/products/speciality_tires/aircraft/candm/care_win01.html

Yeah a little mental carelessness.  Fixed thanks.

1) The spinning wheels will cause vibrations until they are stopped.
2) It is good practice to stop your wheel after you lift off due to flailing tread. Flailing tread can do a lot of damage, especially if it becomes thrown tread. After you stop the tire rotation, it just becomes "hanging tread" and will not do any damage. This is why commercial airlines have systems in place to automatically apply the brakes on retraction and the nose tires get pressed against "spin brakes." Also by learning the practice with fixed gear aircraft, it is more likely to be second nature when you move up to retractable gear.

I was told to do the same thing.
They said that once the wheels started spinning (from take-off) they often didn't stop: the air flow tends to keep them going once they are spinning. It has a slight effect on bearing wear.
I just thought a 2nd reason now: when you land if they are already spinning, then when you land, they don't need anymore energy to get up to speed. But you'd rather they DID require some energy, that would help you slow down. Might be quite small though, I haven't calculated it.
But I doubt that it's a big deal either way.

Aircraft tires on most fixed gear planes generally DO stop spinning pretty quickly. Fly or ride along in an aircraft with wheels visible from the cockpit and you will see them stop pretty soon after liftoff.


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Why are planes slower at higher altitudes?
On many flight simulators, I have noticed that planes tends to get slower with increasing altitude. For example, I can reach 1100 knots just above the sea level in Google Earth flight simulator(F16), but only about 400 knots at 11km with the same plane.

Almost, but not quite, a duplicate of http://aviation.stackexchange.com/questions/5121/why-is-there-a-difference-between-gps-speed-and-indicator-speed.

This is because what you are looking at is the IAS indicator (Indicated Air Speed).
This represents the amount of relative air which flows over and under the wings of the plane. This is what creates lift and enables the plane to fly. This is why this instrument is so important and belongs to the primary flight instruments.
This is not to be confused with GS (Ground Speed), which can be sensed by IRS (Intertial Reference System), or by the GPS. Ground Speed is not relevant to pilot an aircraft, but it's still useful for example to determine the ETA (Estimated Time of Arrival), and for fuel estimation during flight.
The IAS indicator indicates a lower speed because the more you climb, the less air there is. So even the air flows faster, the global pressure feeling is lower.
So, actually planes are not slower at higher altitudes. They are faster (at least most planes whose flight envelope force them to fly faster when they are higher, otherwise they would stall, because they would have to increase their Angle of Attack too much to counterbalance the lack of air flowing above and under the wings). But when they reach their target Mach speed, their speed will decrease slowly as the outside air temperature is decreasing until end of climb.
Sorry for the picture being in French... But as you can see, the flight envelope of an airliner forces him to actually fly faster as it is climbing.

Different speeds
There are different speeds on an aircraft, and it can be quite confusing.

IAS (Indicated Airspeed) is what the instruments directly sense. It is computed by subtracting the static pressure (static ports are placed on the fuselage, vertically to relative airflow) from the total pressure (pitot tubes are placed parallel to relative airflow. This gives a measured dynamic pressure which is then displayed in knots, km/h or mp/h. This speed is showed on most general aviation aircrafts as the main speed indication (but it's actually not a speed and more a 'pressure of air feeling').
CAS (Calibrated Airspeed) is the same as IAS, but without instrument errors. It gives a speed related to the real dynamic pressure felt by the aircraft. This speed is showed on most airliners as the main speed indication (for pilot and autopilot).
TAS (True Airspeed) is the same as CAS when at 15°C, 0% humidity, 1013,25 hPa (29.92 inHg) (ISA, International Standard Atmosphere). This time it is literally the air speed (speed of the air): it is the real speed at which the relative air flows around the aircraft. It takes into account the fact that air density is not always the same. This speed is used when measuring or referring to an aircraft performance during cruise for example. It is also used during flight planning before the wind has been taken into account.
GS (Ground Speed) is the speed of the aircraft relative to the surface of the Earth (the equivalent of the speed showed in a car). In an airplane, we cannot use wheel speed to measure it because we are in the air, so we need GPS or IRS to do this. If there is no wind outside, TAS and GS are the same. If there is 30kt tailwind, GS is 30kt higher than TAS. This speed is used to know when you will arrive at your destination airport, and to know if you will have enough fuel (because the wind is taken into account). This speed is also used when the aircraft is on the ground (when taxiing, or more generally at low speeds when wheels are on the ground). Airlines or airports fix speed limitations when taxiing. The only way to know if you respect the limits is to look at the Ground Speed. Also, the airspeed indicator usually won't be active below a certain speed (45KIAS for the B737).

So, why is the primary airspeed indication (IAS or CAS) the wrong speed (not actual speed) of the air flowing around the aircraft?
This is because this wrong airspeed represents actually the real pressure feeling around the wings that is responsible for the lift. It is this parameter that we need as pilot and it is used in aircraft manuals to describe the V-speeds. It's perfect that IAS and CAS are not corrected with altitude and temperature, because if we know that our airplane stalls at 120kts for example, we won't need to care about altitude or temperature. Our airplane will stall at that same speed regardless of altitude and temperature. If we would use TAS, stall speed would be constantly changing and it would be more difficult to keep the aircraft in good and safe flying conditions.

Here is where you can find these speeds in the Boeing 737 NG cockpit:

I just edited the answer to add a part about the different speeds used in aviation.

In your figure, the horizontal axis is Mach number, with the peak at $0.85$.  As the air temperature falls with altitude, the true air speed corresponding to Mach $0.85$ falls as well.

@Fox In the graphic of moving from TAS to GS, consider replacing "Remove wind" with "Add wind" or "Take wind into account."

@Fox I've made the adjustment Terry suggested to the bottom graphic (mainly because I've always personally disliked "remove wind" myself) - Your original graphic remains here if you dislike the change :)

@voretaq7 : thanks, it's clearer like that!
Besides, I've just added a picture showing where to find these speeds in a B737 NG.
I am considering whether to add Mach & EAS speeds in the list, but I don't know if it's a good idea.

@RossMillikan I've just edited the answer to correct the mistake you mentioned. Thank you!

I've never heard "Estimated Arrival Time (EAT)", I think "Estimated Time of Arrival (ETA)" is more common. Maybe that's just me? How about "Estimated Aliment Time"? ;)

@falstro Thanks, I corrected the mistake!

Not even sure it was a mistake. There are a lot of abbreviations out there. I even think there's an Expected/Estimated Approach Time (the time when you expect to begin the approach), that wouldn't be too far off :)

@jt0dd I would be very interested in this topic.

What you actually see in the cockpit is Indicated AirSpeed (IAS). The airspeed indicator actually measures dynamic pressure (which depends on speed and air density). The instrument assumes that density is 1.225 kg/cubic meter, regardless of altitude. In real life this doesn't happen. As you go higher the density decreases  and so does dynamic pressure. The instrument feels dynamic pressure decreasing and because "it knows" density it's constant it will show you that airspeed is decreasing.

During the climb there is a point where the climb is continued by maintaining a fixed Mach number. This point during the climb is called the cross over altitude.
M=TAS/a

M: Mach number
TAS: True Airspeed
a: Speed of sound

Since the speed of sound is decreasing with decreasing OAT, the TAS has to decrease as well to keep the Mach number constant. However although TAS is decreasing the aircraft is not.
(pprune.org)

What is the source of the (very helpful) graphic, please?

I'm glad that the graph proved to be helpful. I found the image via the following link: https://www.quora.com/What-is-a-crossover-altitude


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What procedures will cockpit crew follow during a sudden depressurization?
When pilots face a sudden depressurization, they will follow some procedures.
What are their sequences and if there is any addressed to the cabin crew?

"OH S$*%@#!..."

In the EMB-145 I flew, the procedure for rapid depressurization was:
                   RAPID CABIN DEPRESSUIZATION

         Aural Warning                    Condition
         ━━━━━━━━━━━━━               ━━━━━━━━━━━━━━━━━━━━
         Voice Message            CABIN ALTITUDE INDICATION
            "CABIN"               HAS EXCEEDED 10,000 FT AND
                                        BECOMES RED
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃ Crew Oxygen Masks ........................................ DON ┃
┃ Crew Communication ................................. ESTABLISH ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛

  Passenger Oxygen ................................. AS REQUIRED
  Maximum Altitude .......................... MEA OR 10,000 FT.,
                                             WHICHEVER IS HIGHER
 ┏━━┓
 ┃IF┃  Emergency Descent Required:
 ┗━━┛  ATC .............................................. NOTIFY
       Emergency Descent Procedure ..................... PERFORM

                              * * *

The priority is getting your O2 mask on and then make sure the other pilot was able to get his on as well and help if needed.  The passenger O2 will automatically deploy if the cabin altitude gets high enough but you can force deployment from the cockpit.  If the depressurization is rapid and/or sudden an emergency descent will be performed to get down into breathable atmosphere ASAP (regulatory requirement time limit).  In the case of emergency descent our procedure did have a PA to make sure everyone was seated and seat belted for the maneuver.
The cabin crew will have their own procedures and while I'm not familiar with them they are equipped with portable oxygen cylinders and have the ability to move around the cabin and assist passengers with masks.


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What's the purpose of yawing?
I've never understood the purpose of yawing, i.e., rotating around the vertical axis by activating the rudder. Provided that airplanes have ailerons that allow banking (used for turning), why do they actually need to yaw? Furthermore, I've read that yawing induces an undesired banking because of the different lifts generated by each wing, due to the fact that one wing "feels" a higher airspeed than the other (a similar effect as a car's wheel spinning faster than the other during a turn). So, if yawing is so "useless" or bothering, why do airplanes have rudders? Moreover, could an airplane takeoff, fly and land without rudder?

related: http://aviation.stackexchange.com/questions/3864 (see in particular the comments)

Take off and fly? Sure. Land? Not in a cross-wind.

yawing induces an undesired banking because of the different lifts generated by each wing - and that's why you use rudder to prevent the yaw so that you do not get the undesired banking.

@Simon, but isn't yawing only induced by the rudder?

@Claudix the ailerons induce yaw in the opposite direction of your intended turn.

No.  Yaw can be caused by the engine, by wind, by the ailerons, ground conditions when taking off and landing, by "weather cocking", etc.  You use the rudder to induce opposite yaw to prevent turning around the Z axis.

I read it as "yawning" and was confused why this was asked on Aviation.SE.

@Mehrdad, in such case that would be a "passengers.stackexchange.com" question

@Claudix: Yeah, I was just praying it wasn't a pilot issue.

Well, this is a simple question. When you are sleepy, bored or tired, you breathe more slowly. Your body requires oxygen and needs to throw out the carbon dioxide. So, your brain makes you take an extra breath, which is deep and long called a yawn, so that you can take in more of oxygen and give out carbon dioxide fully.
All these wrong answers... tsc.

I've read that yawing induces an undesired banking because of the different lifts generated by each wing, due to the fact that one wing "feels" a higher airspeed than the other This is incorrect. The effect you mentioned is indeed similar to differential rotation of car wheels during a turn, but it's negligible for a plane. Yaw is a much more shallow rate of turn, so differential airspeed over wings is almost negligible, easily corrected with slight aileron action.

(continued) The reason rudder yaw induces a bank is because the tail is not aligned with the center of mass. The CoM is in the middle of the fuselage, and the tail (and center of the rudder) sticks up rather high above it. Therefore, you get this slightly diagonal torque-arm acting on the body of the plane instead of a nice straight, perpendicular line.

I think yaw'ing relieves the pilot of tire'ness... :)

@DrZ214: Unless you have a really high yaw rate (for instance, due to a rudder hardover), in which case the airspeed differential does become significant (but, in which case, you have worse problems to deal with).

From a totally practical standpoint, let's say you're in a small tricycle gear aircraft with a freely castoring nose wheel (or a tail dragger) and you're starting your takeoff roll in a calm wind. When you bring the power up, the aircraft is going to want to turn left (why is another question), in other words to yaw about its vertical axis in a counter-clockwise direction, and head for the grass off the left side of the runway. You counteract that adverse yaw by using right rudder to keep the airplane straight. Initially you might have to use some right brake as well until there's sufficient air moving past the rudder. If you had a steerable nose gear, you wouldn't need the right brake.
You reach flying speed, apply up elevator, and lift off. As the airplane rotates about its lateral axis, there'll be some additional adverse yaw as a result of that rotation (again another question). Even if you had a steerable nose gear, that would now be useless as the nose gear is no longer on the ground. Your means of continuing to fly straight ahead is to counter the adverse yaw with rudder, and you would continue to use right rudder as you climb out. If you don't, you'll start turning left.
You reach the altitude you want, level off, and trim for level flight straight ahead. Then you decide you want to turn left. You start the turn briskly with left aileron, but you forgot to use any rudder. If you were looking at the ball on the bottom of the turn and bank indicator, you would see it shoot left. If you had used left rudder while initiating the turn, that wouldn't have happened. You would have started a coordinated turn.
Now you're coming back to the airport, and there's a crosswind from the left. You decide you want to counter that crosswind by slipping into it. In other words, you want the airplane to travel sideways at a speed that matches the crosswind while still having your nose pointing straight down the runway. So, you lower the left wing and to keep the airplane from turning, you use right rudder to keep the airplane aligned with the runway. You match the crosswind by adding or decreasing your bank.
There are other uses of rudder, and especially in aerobatics.
I once flew a few loads of skydivers. The jumpmaster sat behind me, and to help him in positioning for the drop zone, he wanted flat turns. In other words, he wanted the airplane turned while keeping the wings level. So, if he wanted a left turn, that meant left rudder and right aileron. Uncomfortable from a pilot's standpoint.
Finally, back in the 1960s when I was working on my instrument rating, I had an instructor who taught that the best way to track a localizer in close was to use rudder only for small corrections. Personally, I don't like that method, but it's one way of doing things.
A useful thing, the rudder. You need to be able to directly control movement about all three aircraft axes.

Great answer, sir. Missed the answers (short, at least) to those "another questions", though :-)

@Claudix You're free to ask those other questions as other questions. It just fits this site's format better, though, if they're asked as separate questions rather than all answered in the same place. It's easier to find what you're looking for from a search engine and the answers are more concise and readable that way (among other reasons.)

I know :-) However I'll try to find the answer on the Internet first! And by the way, thank you for your detailed answer!

Is "why" because of the right-hand rule?

@JFA, the direction depends on the direction of rotation of your propeller. Most propellers turn clockwise, leading to left yaw

@JFA It's been 55 years since my last Physics class, but as I remember the right-hand rule is simply a convention for representing torque as a vector. You put your right hand around the axis of rotation with your fingers pointing in the direction of rotation. Your extended thumb 90 degrees to the plane of rotation is the direction of the vector. For why the airplane wants to turn left as you bring the power up, check out http://wiki.flightgear.org/Understanding_Propeller_Torque_and_P-Factor.

@Terry I suppose the right hand rule is not a "why" but a demonstration of, as you say, representing torque as a vector. The "why" is the torque. I mostly said it that way because there about a thousand right hand rules, all relating to the motion of objects or particles in a circle.

The purpose of yawing is, as you note, to provide a way to rotate the airplane around a vertical axis.  Ailerons can roll the airplane around a longitudinal axis and produce a slipping turn, and turning with the rudder alone produces a skidding turn, but proper turning is a combination of both aileron and rudder inputs and produces a coordinated turn.
Turning with aileron alone creates adverse yaw when the rising wing provides more lift and more drag than the lowering wing, causing a yawing moment away from the intended direction of turning.  Additionally, if you want to maintain effective gravity downward in your reference frame, you need a yaw moment and rudder is what provides that.
There are many other situations where rudder is essential to flight

Correcting for p-factor turning tendencies
Takeoff and landing in crosswinds
Spin recovery
Controllability in stalls to prevent spin entry
Engine failure in multi-engine airplanes

If rudder were useless, we wouldn't have them.  There are airplanes without rudder pedals, e.g. the ercoupe but this plane still has rudders that act in accordance with the ailerons through turning the yoke to provide the necessary forces for coordinated turns.

Stopping the people in the back puking is also a good use of rudder.

@casey, I think you meant, in the second paragraph, "Turning with rudder alone creates adverse..."

@Claudix no, I meant what I wrote.  See the link and go ask a CFI to demonstrate this to you in an airplane if you are skeptical.

Sorry, I didn't mean to seem skeptical. I didn't know ailerons induced yawing.

don't forget the takeoff/landing roll. Without yawing (whether induced by rudder or nosewheel steering) we'd all have a very bad day :)

There are also situations where yaw alone is directly useful to introduce intentionally uncoordinated flight, for example, a slip to landing. Fly a tailwheel airplane, maybe even an old one with no flaps, into a short field and this is de rigeur for achieving a steeper descent profile without gaining energy and airspeed. It's also how to correct for a crosswind on approach and landing. As a CFI, that's the first answer I'd think of when reading the OP's question.

The primary purpose of yawing is to stop yawing.
Airplanes with long wings have a tendency called adverse yaw. Adverse yaw causes the plane to yaw away from the direction of turn when attempting to turn using ailerons and elevator. The more you try to turn left, the more the nose point right.
The big problem with adverse yaw (apart from it being potentially confusing to pilots) is that while the lift from the wings held at a bank angle is trying to turn the plane in one direction, the nose is going in the wrong direction eventually causing the plane to stall. This can make turning impossible.
Early aerodynamicists didn't fully appreciate adverse yaw because experiments with model planes tended to have the plane fly straight. It's not until you attempt a turn that you'd encounter adverse yaw. Even early glider pioneers didn't fully appreciate it much because a lot of the early hill hopping glider flights were also in straight lines.
Among the first people to encounter and attempt to solve the problem of adverse yaw was the Wright brothers. Taking inspiration from other early glider pioneers and large kite fliers their early gliders didn't have a rudder. They found that it was unstable when the wings tip to either side (bank). What they experienced was adverse yaw. Their solution was to add a vertical stabilizer and it solved the problem for the most part. Later they made the rudder movable to allow the pilot to further correct for adverse yaw manually. We still teach this "correction" maneuver - it's called the coordinated turn.

I'd like to note that these days there are other ways to overcome adverse yaw including (but not limit to) aileron differential, drag rudders, spoilerons (technically a variant of drag rudders when used this way) and low aspect ratio wings. It's just that the rudder was invented as a solution to adverse yaw.

A turn performed with just the ailerons and elevator will not be "coordinated". This is to say, the apparent G force will not go straight "down" through the aircraft's floor, which is desirable for various reasons. Among them is passenger comfort (being pulled down is less unpleasant than being pulled sideways), but there are other advantages too.
In addition to making coordinated turns, the rudder is useful to correct yaw induced by gusts of wind, to set up and maintain a wing down attitude in a sideslip crosswind landing, to quickly lose altitude without picking up lots of speed (somewhat notably used by the Gimli Glider), and for various aerobatic maneuvers.

To my knowledge, at least during combat/ww2 Using the rudder to yaw  in that way was often applied by pilots who were weary of being attacked from behind. Adding a  small amount of rudder/aileron to yaw, meant that instead of flying straight and level, the aircraft would sideslip through the air, therefore throwing off any enemy fighters aim/Bullets. Any combat airman will tell you, never ever fly straight and level in a combat zone...

Welcome to aviation.SE! This is an interesting answer but yawing also increases drag and reduces speed, which would presumably be a disadvantage in a dogfight. Do you have any sources you can link to that mention this technique? And if you're new to the site, the [tour] may be useful.

Moreover, could an airplane takeoff, fly and land without rudder?

Absolutely.  I have an electric ducted-fan model of an F-16 that has ailerons, elevators, and a steerable nosewheel, but no rudder.  It takes off, flies, and lands just fine, and this concept could certainly be scaled up to a full-scale airplane.
In many light airplanes, much of the time the rudder is used more of a "trim" control to optimize streamlining by cancelling out effects like p-factor, but is not absolutely essential.  It becomes more important in crosswind landings, but some light airplanes like the Aercoupe have been built with no independently controllable rudder.  Such aircraft invariably have tricycle landing gear rather than tailwheel landing gear, for greater directional stability on the ground.
In sailplanes (gliders) with long wings of high aspect ratio, adverse yaw is very pronounced and it is very important that the rudder be applied along with ailerons when entering and exiting turns.  There are various design features, like differential aileron deflection, that can reduce adverse yaw.

There are two main reasons: control on the ground and efficiency in flight.
When an aircraft is rolling on the ground, the only way to control its direction is with the rudder. When a pilot both takes off and lands an aircraft they use the rudder to steer. If the aircraft had no rudder, it would likely roll off the runway.
In flight, the rudder is used to keep the fuselage parallel to the line of travel. For example, if the pilot makes a turn without using the rudder air will hit the side of the aircraft and the drag on the plane will be significantly higher.
Another cause of inefficiency in propeller aircraft is what is known as P-factor. This is a yawing of the aircraft due to asymmetrical force in the propeller causing drag. The pilot must hold down the rudder constantly to prevent this. It can become quite tiring to do so in long flights.


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Can any aircraft take off using auto-pilot?
So there are some types of aircraft that can land under auto-pilot (or is that a trope?). Can those same planes go from stop (at the gate) to in-the-air flight without a pilot actually touching a control column/flight-stick?
Caveats: I'm not talking about UAV's. Also, I'm not concerned about "would you," but rather "could you."

There are two very different parts in your question. Take off and taxi. While CAT II equipped aircraft can land under autopilot and CAT III equipped aircraft can land and roll out under autopilot, there are no aircraft that could taxi under autopilot.

Correct.  Taxiways are a lot narrower than runways. This is assuming today's technology.  Theoretically this could happen but the ramp area is a very busy place.  A set of human eyes is a vital safety measure.

If you are asking if a plane can be taken from the gate all the way to the sky by auto-pilot I'm quite certain the answer is no.  Not because a plane can't configure itself for take off and power up the engines (I'm quite certain it can, though I would defer to more knowledgeable people for that).  Not for that reason, but rather because there is no system in place to electronically guide an aircraft through a series of taxiways all the way to the base of the runway so that it can line up to take off in the first place.
Keep in mind, ATC gives a lot of instructions for ground operations, funnelling aircraft this way and that via taxiway A through Z.  They'd have no way of telling an auto-pilot about these directions, there's no computer to send out signals and there is no voice recognition build into the plane.  If you told the pilots, they would have no way to input the instructions into the autopilot, nor would the autopilot know what to do if it ever received such instructions.  It just isn't designed to follow taxiways.
Also, beyond just taxiway layout, there are a lot of planes in close proximity on the ground, plus bunches of other important vehicles (fire trucks, fuel trucks, de-icers, etc.)  Steering around all of those, faultlessly, would be quite the feat.  We've only just gotten to the point where a vehicle can drive across open terrain without crashing itself,  I think a plane navigating an airport (and doing it while minding ATC) is a bit of a ways off.
EDIT: Raidri's video does a much better job of showing the current state of driver less vehicles, and his comment is correct. I did underestimate how good the systems are these days.
That being said, I'm not changing my answer.  There still, currently, is no way to convey ATC instructions to the aircraft.
As a point of clarification: I saw the question as asking about technology in it's current state.  Will we eventually be able to do this.  Heck yes, absolutely.  A great deal of the technology is in place and what's left is either right on the horizon or close to it.  FAA clearance will probably take some time, but we'll get there.  I was mainly answer the question from the perspective of modern tech and what is currently ready to be installed on a plane.

I think you underestimate the current status of driverless cars: https://www.youtube.com/watch?v=dk3oc1Hr62g. An airport yould be more city streets like open terrain.

@Raidri Wow, good catch, thanks :).  After driving 6 hours yesterday and looking forward to another 4 today all I can say is this: sign me up.

"could" you : yes. After pushback (and away from the gate area in general) airports are more-or-less obstacle free. Modern GPS is plenty accurate enough to follow a taxiway sequence up to the hold-short line (this is a much simpler problem than driving an automated car around town [1]). A cheap rangefinder in the nose would prevent goosing the plane in front.
After the plane is on the active runway it's just a matter of configuring a few things and running the power up. No problem at all with today's technology.
"would" you: no. Such a system would be rather expensive, someone has to program it, and it provides no particular advantages over having the pilot steer it following a $5 map, or at smaller airports just the signs beside the taxiway.
[1] Support vehicles at an airport are not really an issue: aircraft have absolute, unquestioned right-of-way in much the same manner as freight trains. Even the fire department waits for the tower to approve crossing a runway.

Not to nitpick too much, but there's a huge difference between "has right of way" and "all objects always get out of he way all of the time."  If you're using a computer program with no cross checking, it had better be the later that's true.  As a collegue of mine is fond of saying, "never overestimate the user", I think we have to assume that, at some point, someone will be in the way, probably on accident but still...  I think your underestimating the complexity this system would need.

Collisions between taxiing aircraft and ground vehicles happen already, plus the occasional blowover from someone taking the access road right behind the A380 spooling up on the active. The anti-goosing sensor might even reduce the accidents. The question asks "could you..." today and you can. Not with today's aircraft (which are about 20 years old technologically) but with a complete, brand-new system. I do not expect to see one this century outside of a droneport.

"and it provides no particular advantages over having the pilot steer it following a $5 map". It offers the same advantages that auto-land offers - the ability to continue operating during reduced visibility.

If the visibility is so low that you can't follow the white line then you won't be leaving the gate area, possibly due to lack of passengers if the same fog closed the local roads. Yes, CatIII can autoland in dense fog but landing is not an option. Takeoff is.

No aircraft flying today can takeoff by itself.  Even "auto land" requires pilots to set up the "Flight Management Computer", position the aircraft to intercept the final approach course,  and then physically operate the flap lever and landing gear lever.

@MikeSowsun as i said, today's aircraft are technologically old. Flaps and gear can be automated very easily. We have chosen not to do so.

If you're simply asking "does this exist today?", then the answer is simply no.
I believe there are no automatic take-off systems currently.
Don't forget drones are flown by the remote human pilots; I don't think even they have an automatic takeoff system.
Regarding "would it be technically possible as an experiment?", Paul has explained the situation.

There were two reasons I suggested not to consider drones/UAVs. First, some would say they are remotely controlled, so there is a pilot, like you said. I also know that they can be programmed to follow flight paths/patterns, I just don't know how much of their flight can be set up to be auto-controlled (via GPS, etc). Second, they are (very soon, if not now) going to be fully automated (such as what Amazon is planning). This is worrisome, at least to me, in that we will (likely) have multiple aircraft, with only a few monitors, flying through low airspace.

Just to add insight, Drones take off and land under local pilots control, (delay involved iirc), and inflight they are passed to the people in the ever so famous shipping containers afterwards.


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What kind of liquid can be seen 'leaking' from the bottom of a Boeing 737-800 engine?
I was on holiday over the Christmas holidays, and was sat in the airport awaiting our plane back to England, looking out of the large window which over looked aircraft that have just landed and the passengers get of, when a Boeing 737-800 turned up, looking at its left engine (I could not see the right engine from my viewpoint), I saw a clear liquid leaking from the bottom of it, what was this liquid, and were might it of come from?

source: wikipedia

I have circled were I think the liquid came from on the left engine in the picture above.

It was the tears of all the flight crew who you made work over christmas!

If you could point out exactly where the liquid was coming from, that could help. The nacelle has drains for condensation, fuel, hydraulic fluid, and engine oil. If you're sure it was totally clear that narrows it down.

Feel free to add it to your question

Probably condensation, especially if the engine was hot from running and it was a coolerish day outside. Just like your car leaking water (condensation) from it's engine/ac system when you park it.

Related: CFM56-7 drain system

I can verify the "back to England" part after reading the last sentence of the first paragraph.

There is really no telling without being there & getting a sample of the fluid.  A mechanic can tell the differnce between hydraulic fluid vs engine oil vs fuel vs etc, but the place where any of them would come out is essentially the same spot under the engine.
In general, any of the above is uncommon but far from unheard of, and in small amounts is no big deal.  The fittings aren't supposed to be leaking, so a mechanic will probably end up fixing whatever it was, but the hazard from a few drops just isn't all that great.  Sure, the idea of "oh, just a fuel leak, no big deal" intuitively sounds pretty bad, but the fact of the CFM engine is that the motor is incredibly tolorant of this sort of thing with nothing bad actually happening.  It needs to be checked out because something isn't exactly right, but there would have to be several other things go wrong as well (each of which is pretty rare) before anything bad actually happens.
Some lines are allowed no leakage at all, while others actually do have a "drops per minute" limit for what is allowed without needing maintenance action.  Not being a 737 mechanic, I couldn't tell you offhand which fluids are in which category.  If I see it leaking, I call the mechanic & he can sort it out.
Of course, in the days of the big radial engines, the saying was that when it's leaking, that means there's still fluid in it, and when it stops dripping, THAT is what's bad, because that means you don't have any more oil!
We're a long, long ways from those old days!

It is okay to ask the question "What is that fluid," but do not under any circumstances catch a drop with a finger and test it with your nose.
The A7E was the light attack aircraft aboard US carriers during its day. As an older aircraft it had what we called a "piss tube." Newer aircraft have greener solutions to the problem of human waste. The piss tube was a funnel secured below the pilots feet in the cockpit that terminated at a small outlet hole on the fuselage. While flying, the Venturi effect created a soft vacuum that pulled the urine outside the aircraft.
I never used it because you had to do all sorts of contortions. You were in a 4-point harness with a g-suit, your bulky survival vest, and then the flight suit. A squadron mate of mine on the other hand was an expert at using the technology.
He was on the deck one night turning and getting ready to launch when he decided he had to pee. The Plane Captain had all his checks done and he was just waiting. He pissed in the tube and shortly after that the Plane Captain was motioning him. His signs suggested that he had a leak from the fuselage. The pilot waved the approaching Plane Captain off frantically, but to no avail. The Plane Captain caught a bit of the liquid on a finger and brought it to his nose. Case solved.

That's just known as "water sports" to some! And in a desert survival class I took, they took great pains to remind us that urine is sterile and perfectly safe to drink.


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What license woud an ex-military aircraft (E.g, L-39) go under according to the UK-CAA, an EASA licence or a non-EASA license?
I've been looking around for the sale of ex-military jets, to own and fly, namely an Aero Vodochody L-39 Albatros, and so out of interest had a look at the CAA website for infomation on licenses, training and general ownership etc, and across the terms EASA and non-EASA licenses, I rougly understood the meaning of them, but am unsure as to which license I would need to own and L-39.
Whilst asking this, I would be appreciative if anyone could give me some general tips and advice on owning an ex-militatry jet trainer and the kind of courses/training I'd have to undertake to own and fly it. Thank you in advance.
Please edit if I have left something important out or anything like that.

Related

According to CAP 632

6.1 Ex-military aircraft on the UK register with a ‘Permit-to-Fly’ may be piloted by either private or professional licence holders. Pilots must hold a current civil licence with a current civil class or type rating appropriate to the ex military type[s] being flown or where no civil type rating exists, an Aircraft Type Rating Exemption from the need to hold a type rating.
   Any questions relating to licence matters should be addressed to CAA PLD

...

Any turbine-powered aeroplane - To fly an ex-military turbine-powered
  aeroplane a pilot must hold an Aircraft Type Rating Exemption. See paragraph
  6.7 for details of obtaining an Aircraft Type Rating Exemption

and so on.
Ex-military aircraft are non-EASA aircraft. Ref

So the aircraft has to be ex-british and I would have to have flown it in the military beforehand, or be a commercial pilot?

@anonymous. 1) AFAIK non-British aircraft can be registered in the UK and many are. 2) No. 3) No.

Thank you very much for your clarification and original answer, I appreciate it.


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What can cause a sudden jump in Indicated Air Speed?
My aircraft heading is 321 with constant Static pressure, wind speed and steadily increasing Ground Speed.
Meanwhile, there was a sudden jump of 10kts in IAS from 140 to 150 kts withing a span of 1 sec.
For this span, Wind direction is rapidly changed from 176 tp 300 degree A.
Is wind direction is cause for IAS jump ?

Related

Yes, exactly, the change of wind direction is the cause for your sudden increase of IAS.
I have marked your aircraft heading with a magenta arrow and the direction the wind comes from with a brown arrow. So, you are going 140 KIAS at this moment and you have a steady 5 knot tailwind. That means you have a GS of roughly 145 kts (we will ignore air density and that it's not a perfect tailwind in this example).

Now the wind direction changes all of a sudden.

You have now got a nearly direct headwind. That means there are in total 10 kts more worth of air going into your pitot tube and thus indicating 150 KIAS. That's because your aircraft lost the 5 knots tailwind and at the same time got a 5 knot headwind. Because of the fast wind direction change, the aircraft kept its "momentum" and the airspeed increases by double the wind speed.

Can you explain little more : " How tailwind and headwind both can contribute to increase in Indicated airspeed ? What I think only TAS should be affected , not IAS !

@VijnanaYogi When there is wind suddenly hitting the plane from the front, like a windshear, there is more air going over the aircraft and also the pitot tube. Same with tailwind, but there is less air and as such a decrease in IAS.

Yes, But in above explanation ( That means there are in total 10 kts more worth of ....) its seems tailwind is also causing increase in IAS , that's point of confusion ....

@VijnanaYogi The tailwind itself does not increase IAS, only it's sudden decrease. If tailwind decreases, IAS increases by this value, because of the plane's inertia. In your case the tailwind completely disappears, hence the first 5 knot increase in IAS. At the same time the aircraft experiences a 5 knot headwind, again increasing IAS by 5 knots, to a total of 10 knots. It's only because of the planes inertia.


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What is meant by a fused radar track?
Can anyone explain what a fused track is from the perspective of radar?

I believe it's where data is aggregated from multiple radars and tracks are correlated with each other.

Don't understand why this has so many downvotes - more context would be nice but is hardly essential.

A fused radar track is a track (series of radar detections belonging to the same aircraft) that contains information from detections by multiple radars.

Creating a fused track seems a simple thing to accomplish, however it is not as easy as it may seem.
First of all, lets look at a mono-sensor track. That is a series of detections by a single sensor belonging to the same aircraft. The tracking system has to determine for each detection to which track it belongs to associate the flight data (e.g track label) for that track to the plot (display of the detection on the screen). For primary radar systems (and to lesser extent Mode A/C radar) this can already be difficult when tracks are crossing each other at shallow angles. Especially when you know that the measurements are a bit noisy; an aircraft flying a straight line does typically create a somewhat randomly jagged line.
Now suppose we want to add a second radar to the mix, because the coverage area of a single radar is not big enough. Now we have a situation where there is a common coverage area where the two radars provide overlapping coverage.
In an ideal world, this would show the detections exactly in line with each other. However, each radar has their biases are other measurements error (with various sources, partly external to the radar system). So instead of one track for one aircraft, we end up having two tracks for one aircraft, sometimes a mile apart. This is of course not ideal and can even lead to safety risks.
A simple solution is to have a mosaic system; in such a system there is for each geographical area exactly one radar appointed to provide the surveillance data to the air traffic control's display. The down side of such a solution is that at the edge of the mosaic tiles, when the aircraft moves from one radar area into another, the track may jump because of the differences in measurement errors in both radars. Again not an ideal situation.
That is where radar fusion comes in. In a multi radar tracker, data from multiple radar is combined into a single track. In the process, biases and other measurements errors are greatly reduced, resulting in a track that generally much smoother than a single sensor track. Because of the identification of radar biases, there is no discontinuity (jump) when track move out of coverage of one radar. And due to the higher rate of updates from multiple radars, the update on the radar screen can often be increased as well.
The fused radar track is no longer a depiction of detected positions, but calculated estimate of where the aircraft is based on data from all radar (or other sensors) used in the fusion process. The mathematical process behind the fusion often uses on a form of Kalman Filtering, highly adapted for this task. Outliers are discarded to improve the smoothness of the track.
Trackers like ARTAS or Phoenix do not only fuse radar data into the track, but also include data from ADS-B and Wide-Area Multilateration (WAM). We then speak about a (fused) multi-sensor track instead.

Relative to commercial flight tracking, what this means is that observations from different air surveillance zones are combined to provide a single track of motion for the flight. For example, the major mode S sensing radars in the United States have the following sites:

As you can see there are many different radars. To track a flight that moves across the country, the information from many different radars has to be combined and correlated. This is called "data fusion".

Citing references would improve this answer.

I also wonder—regarding your opening clause—how the commercial aspect of a flight relates to this topic?

@JonathanWalters In military contexts the term "fused track" can have different connotations than what I have explained in my answer. However, my guess is that the OP is asking about the fused tracks of civilian air carrier vehicles.

I see. I would suppose "civil aviation" to be a better differentiator in that case since civil aviation radar tracking includes private, corporate, charter, and airline flights regardless of the commercial nature of the flight.


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How to select and electric propulsion system for my experimental Ultralight?
I've been trying to design and build, my experimental Ultralight using electric propulsion system, My plane designed base on Slaler and Gensis ultralight , following is the primary calculation
Empty weight 120KG
Wingspan 9.1
Length 2.8
cruise speed 120 Km/h
propulsion 20kw
I'm trying to select a pair of Brush-Less Direct Current motors (BLDC) for my airplane, but I have no experience using BLDC's before, whereas there is a lot of BLDC ambiguous options (mostly in Aliexpress) which claim to be a different KW.
My design goal is to find the best possible options for propulsion to have longest possible flight time duration.
Following is the 3D image of my design (located in my GrabCAD account)
any advice really appreciated

I can't help you with the motors, but what kind of calculations is the design based upon? I'm asking because the tail assembly seems to be way too small.

The tail boom also looks beyond flimsy. Both of the designs you are taking inspiration from had much more substantial tail structures AND required less torque from the HTP due to the CG being very close to the wing. Your design moves the CG forward with that long (2 crew?) cockpit while weakening the tail.

Thanks for your comment but I've tried to  move CP to CG as close as possible to reduce the amount of tail volume and reduce wight and negative lift by tail

FLÜGEL World Directory Of Light Aviation 2019/2020 - English PDF could be a good starting point. On pages 278 - 281, a total of 71 lines of electric motors from 42 manufacturers are featured. Disclaimer 1: some data fields are incomplete for many of the entries. There are German, English French and Chinese versions in paper and pdf, starting at €8. Here's the paper version in German:
Welt-Index Ultraleicht und Flugzeug 2019/20 Wings of the World
In any version, the aircraft listings themselves may also be of interest, as aircraft models which are available with electric motors as the primary powerplant are highlighted with a green e icon. In browsing those, you may glean some emperical inspiration in terms of electric motor vs aircraft size. Disclaimer 2: the power-plant details for the aircraft listings are not always complete.
The free annual e-flight Journal by the same publisher includes motor manufacturer adverts which may be of some use.
Lastly, at the end of the online Kitplanes 2020 Engine Buyer’s Guide are four electric motors from one manufacturer.
Final Disclaimer: I have no vested interest in these magazines, publisher or any products listed or advertised.

The rated power of an electric motor can vary wildly; it's not a fixed quantity like an IC engine. If you fit a larger prop, or run it at a higher voltage, it will draw (and hopefully produce) more power. However it'll also produce more waste heat, and may well be less efficient.
You can run it way beyond the rated power for a few seconds, or overheat a motor way below its rated power if there isn't enough cooling airflow.
Ideally you want a chart showing the efficiency for a range of speed and power. Then you need a chart for your proposed propeller, showing the thrust produced at a range of rpm, and the power required to reach that speed.
Then you can set out some design requirements, perhaps static thrust for take-off, climb and cruise performance. Look those up in the charts, and see if the motor will be operating efficiently, or if you're pushing it beyond its limits.
In the model world, you can usually use someone-else's project as a starting point, but some bench-testing and experimentation is required to find the best combination.